/*
    * Copyright 2011-2013 smartics, Kronseder & Reiner GmbH
    *
    * Licensed under the Apache License, Version 2.0 (the "License");
    * you may not use this file except in compliance with the License.
    * You may obtain a copy of the License at
    *
    *     http://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package de.smartics.util.lang;
  
  import java.io.IOException;
  import java.io.InputStream;
  import java.net.MalformedURLException;
  import java.net.URL;
  import java.util.Enumeration;
  
  import org.apache.commons.lang.ObjectUtils;
  
  /**
   * A constraint on resources to be loaded from the class path.
   */
  public final class ClassPathContext
  {
    // ********************************* Fields *********************************
  
    // --- constants ------------------------------------------------------------
  
    // --- members --------------------------------------------------------------
  
    /**
     * The class loader to load the directory listings.
     */
    private final ClassLoader classLoader;
  
    /**
     * Selects the archive root to load from.
     */
    private final String archiveRoot;
  
    // ****************************** Initializer *******************************
  
    // ****************************** Constructors ******************************
  
    /**
     * Default constructor.
     *
     * @param classLoader the class loader to load the directory listings.
     * @param archiveRoot the value for archiveRoot.
     * @throws NullPointerException if {@code classLoader} is <code>null</code>.
     */
    public ClassPathContext(final ClassLoader classLoader,
        final String archiveRoot) throws NullPointerException
    {
      Arg.checkNotNull("classLoader", classLoader);
  
      this.classLoader = classLoader;
      this.archiveRoot = archiveRoot;
    }
  
    // ****************************** Inner Classes *****************************
  
    // ********************************* Methods ********************************
  
    // --- init -----------------------------------------------------------------
  
    // --- get&set --------------------------------------------------------------
  
    // --- business -------------------------------------------------------------
  
    /**
     * Constructs the URL to the resource.
     *
     * @param resource the resource whose URL is requested.
     * @return the URL to the resource or <code>null</code> if the resource cannot
     *         be found on the class path.
     */
    public URL getResource(final String resource)
    {
      if (archiveRoot == null)
      {
        return classLoader.getResource(resource);
      }
  
      // TODO: maybe we should simply construct the URL?
      try
      {
        for (final Enumeration<URL> en = classLoader.getResources(resource); en
            .hasMoreElements();)
        {
          final URL current = en.nextElement();
          final String urlString = current.toExternalForm();
          if (urlString.startsWith(archiveRoot))
         {
           return current;
         }
       }
     }
     catch (final IOException e)
     {
       // return null
     }
     return null;
   }
 
   /**
    * Opens the stream to the resource.
    *
    * @param resource the resource whose stream is requested.
    * @return the stream to the resource or <code>null</code> if the resource
    *         cannot be found on the class path. If a stream is returned, the
    *         client is responsible to close that stream.
    */
   public InputStream getResourceAsStream(final String resource)
   {
     final URL url = getResource(resource);
     if (url != null)
     {
       try
       {
         return url.openStream();
       }
       catch (final IOException e)
       {
         // return null;
       }
     }
     return null;
   }
 
   /**
    * Constructs the URL to the resource.
    *
    * @param resource the resource whose URL is requested.
    * @return the constructed URL.
    * @throws IllegalArgumentException if the URL to the resource cannot be
    *           constructed.
    */
   public URL createUrl(final String resource) throws IllegalArgumentException
   {
     try
     {
       final String urlString =
           (archiveRoot != null ? archiveRoot : "") + resource;
       return new URL(urlString);
     }
     catch (final MalformedURLException e)
     {
       throw new IllegalArgumentException("Cannot construct URL with resource '"
                                          + resource + "'.", e);
     }
   }
 
   // --- object basics --------------------------------------------------------
 
   /**
    * Returns the hash code of the object.
    *
    * @return the hash code.
    */
   @Override
   public int hashCode()
   {
     return ObjectUtils.hashCode(archiveRoot);
   }
 
   /**
    * Returns <code>true</code> if the given object is semantically equal to the
    * given object, <code>false</code> otherwise.
    *
    * @param object the instance to compare to.
    * @return <code>true</code> if the given object is semantically equal to the
    *         given object, <code>false</code> otherwise.
    */
   @Override
   public boolean equals(final Object object)
   {
     if (this == object)
     {
       return true;
     }
     else if (object == null || getClass() != object.getClass())
     {
       return false;
     }
 
     final ClassPathContext other = (ClassPathContext) object;
 
     return ObjectUtils.equals(archiveRoot, other.archiveRoot);
   }
 
   @Override
   public String toString()
   {
     return archiveRoot != null ? archiveRoot : "<unspecified archive root>";
   }
 }